Exceptional care must often be taken to protect
personnel and ESD-sensitive parts from the consequences of improper
grounding. To help deal with grounding problems in the field, here
is advice form a member of the ESO/ESD Grounding Standards Committee.

Grounding sounds simple, yet there
are many considerations that must be dealt with when we attempt
to solve electrical overstress and electrostatic discharge (EOS/ESD)
problems by using proper grounding in and around static-sensitive
devices.

Ironically, after building such EOS/ESD-sensitive products as laser
printers, computers and copiers in plants with total ESD control,
these products usually operate in ESD-hostile environments for the
rest of their lives. When latent failures, power surges and ESD
events occur in use and cause failure or malfunction, a field-service
technician is called to the site to make repairs. He or she may
be required to service a product that is not located on the forty-fifth
floor of a high-rise office building. As a result, to control ESD
and field effects the technician may encounter, he or she becomes
dependent on equipotential or floating grounds of uncertain characteristics
unless there is safe, positive access to an earth ground of known
quality.

This article discusses the use of the equipment ground circuit of
a building as a nonfloating ground in field-service situations,
where zero or near zero static voltage levels are required. The
scope is limited to situations and locations where, for practical
purposed, no AC frequencies above 60Hz are within reach of a field-sevice
technician.

Although the term impedance is occasionally used throughout
the course of this article, it is used parenthetically in order
to call the reader's attention to areas of importance to personal
safety, in which higher AC frequencies are encountered as a practical
matter, the formula that is derived form MIL-STD-454 (Ref 1) does
not apply.

Resistance to Ground Limits

Webster's Dictionary (Ref 2) provides
three definitions of an electrical ground: A. The position or portion
of an electrical circuit at zero potential with respect to the earth;
B. A conducting connection to such a position or to the earth: C.
A large conducting body, such as the earth, used as a return for
electric currents and as an arbitrary zero of potential.

In definition C, we are confronted with the fact that large conducting
bodies may only provide an arbitrary zero potential. Too often we
incorrectly assume that when we connect the ground cord from a static-controlled
workstation to a cold-water pipe, we have provided sufficient grounding
to prevent ESD and EOS damage.

According to the Navy Electrostatic Discharge Training Manual, "In
order to limit residual voltages caused by static generation at
a typical ESD-grounded workbench to approximately 10 V, the maximum
resistance to hard ground should not exceed 10 M Ohms" (Ref
3). This upper limit was presumably set to establish a two-second
decay time for an electrostatic discharge from a person and is based
on the RC time constant used for the human-body model as shown in
FIG 10-1 of that document. The minimum resistance to ground is governed
by personnel safety considerations.

Soft vs. Hard Grounds

To reduce the hazard of severe electrical
shock, any conductive worksurface must be properly soft-grounded.
A conductive worksurface should not be directly connected to a hard
ground, that is, a connection to ground through a path that has
little or no resistance.

[EOS/ESD Technonlgy Editor's Note: The nature of the application
must be taken into account when determining the need for series
resistance to ground. Mr. Hynes is accurate with respect to field-service
situations in which the presence or absence of 60-Hz power on objects
in the work area is unknown. Obviously, in such a situation, personnel
protection must take first priority because the uncertainties in
the work situation place personnel at risk. However, in the case
of a workstation in a controlled environment such as a factory,
where proper precautions have been taken in the control of 60-Hz
power, and where regular inspections take place and monitoring techniques
are used, the likelihood of damage to components may be greater
than the risk to personnel, and harder grounds are more permissible.
In the latter case, the desired charge decay rate can become the
controlling factor.]

All possible parallel paths to ground- even those beyond the immediate
work area- from people, metal furniture, electrical equipment, floor
mats, table mats, wrist straps, etc., must be considered.
An unprotected paralleled path to ground
could reduce the resistance needed for personal protection. As one
example, an uncovered stud on the underside of a table mat or field-service
mat can become an unimpeded parallel path to ground if the mat or
field-service kit is placed on a grounded metal table or conductive
computer cabinet. Also, try to keep one hand in a pocket whenever
working around unknown voltages.

Alternatively, too much resistance to ground will affect the static-decay
rate of the worksurfaces, and the worksurfaces will not drain static
electrical charges within the interval required for safe handling
of sensitive electronic devices.

As opposed to a hard ground or an overly resistive ground, a soft
ground is mandatory. A soft ground is a connection to ground through
a resistance high enough to limit DC current flow to less than 5
mA. The resistance needed to achieve a soft ground is dependent
upon voltage levels and AC frequencies that could be contacted by
personnel near the ground.

Under normal circumstances, where only 110-V, 60-Hz, AC power sources
are within reach of a person, an absolute minimum of 250,000 Ohms
resistance (impedance) should be used.

If higher voltages sources or higher frequency voltages are within
reach, the resistance (impedance) needed to achieve a soft ground
must be calculated. Where only 60-Hz AC frequencies are present,
the following formula may be used:

Highest V Within Reach
=

5 ma (.005)

Minimum Resistance Needed
to Achieve Safe Ground

Based on this formula, a 1-MOhm resistor
provides ample safety margin, even if 220-V, 60Hz AC power sources
are within reach.

Periodic checks should make sure that an electrically conductive
leakage path, limited by the proper resistance, exists between the
wrist strap and ground and between the floor or table mat and ground.
All ground cords should be examined for wear and tear, and all resistors
should be checked on a regular basis to make sure they are functioning
properly.

Single Module Provides Safety,
Power, Grounding

The Model FS-1 provides a field-service
technician with safe AC power and assured grounding in a single
package. It includes AC power protected by both a Ground Fault
Circuit Interrupter (GFCI) and a circuit breaker and also includes
an outlet-polarity tester (using a green LED). When snapped
to a worksurface or mat, it provides a confirmed soft-ground
connection. Its banana jack also is soft-grounded and accepts
any standard wrist-strap coil cord. Touching a test button flashes
an amber LED and sounds a tone if the technician is properly
grounded. The FS-1 costs $119.50.
Pilgrim Electric Co., 105 Newton Road, Plainview, NY 11803 (516)
420-8990.

Circle
49

Common-Point Grounding Systems

Figure 1. Common-point grounding per DOH-Handbook
263.

Two optional wiring diagrams are provided. Fig
1 is the diagram as recommended in DOD-Handbook-263 (Ref 4). Note
that the wrist strap and the table mat are connected in series to
ground, while the floor mat, through its own resistor, is connected
in parallel to ground. In effect, this diagram provides 2 MOhms
of resistance to ground for the person wearing the wrist strap and
1 MOhm of resistance to ground form both the table mat and floor
mat.

In order to make the series connection
between the wrist strap and the ground cord, newer wrist straps
and ground cords with stackable snaps are available (see Fig 2).
These snaps have female and male studs on opposite sides of the
snap.

Figure 2. Newer wrist straps and ground
cords offer stackable snaps.

In Fig 3, individual connections form
a wrist strap, table mat, and floor mat, each through its own current-limiting
resistor, are connected to a bus bar or common-ground junction box.
This method ensures more rapid static decay from the person than
does the series-parallel diagram in Fig 1. Either method is acceptable
so long as total resistance connected in series does not exceed
10MOhms.

In Fig 4, we see a third method. This
method of connection puts the current-limiting resistors of both
the wrist strap and ground cord in series with the resistance of
the table mat or covering. Since resistances in series are cumulative,
it is possible that total resistance to ground could be so high
that static-decay time from the person wearing the wrist strap might
exceed acceptable time limits. (Note: Mats equipped with studs at
both sides are intended for left- or right handed use for the worker's
convenience, not for making series connections; see Fig 5.)

Figure 3. Individual grounding
connections from a wrist strap, table mat, and floor mat are
connected to a common ground through separate current-limiting
resistors.

Figure 4. This method
of connection puts the current-limiting resistors of both the
wrist strap and ground cord in series with the resistance of
the table mat or covering.

Figure 5. Don't make series connections with
the alternate snaps provided by some worksurface makers at the
right and left sides of their products.

Voltage Decay vs. Time

On one hand, the lower limit for resistance
to ground is a matter of personal protection against possible shock
or electrocution, should a defective power cord or piece of equipment
come in contact with a worksurface or person. On the other hand,
the higher limit is dictated by the damage susceptibility of the
ESD-sensitive device.

From FED-STD-101, Method 4046 (Ref 5) and Mil-B-81705 (Ref 6), we
find that a time limit of less than 2 sec has been established for
discharging a +/- 5,000 V charge to 0 V when testing packaging material
used for ESD-sensitive items. In theory, and probably backed by
some obscure time-and-motion study, it takes a person at least 2
sec to pick up a package, open it, reach in and touch the device.
This same time limit has been extended to discharging a person who
has to work on a unit in the field.

ESD Waveform

However, as shown in MIL-STD-883, Method
3015 (Ref 7), we find that the decay rate is exponential (see Fig
6a). In fact, the complete discharge of a 5-kV charge may never
reach zero if the earth itself is "an arbitrary zero of potential."
Thus, depending on the quality of a ground at any given point, some
residual voltage may remain on a grounded person or worksurface
(Fig 6b).

Residual Voltage vs. Time

In order to limit residual voltage
caused by tribolectrification and changes of capacitance at a typical
ESD-grounded workbench to approximately 10 V, the maximum resistance
to a a hard ground should not exceed 10 MOhms (Ref 2, p 120). Thus,
all series resistance to hard ground must be considered, and the
sum of the various resistances must be known (see Fig 7).

If we now add the resistance of a could-water pipe and its connections
to the ground circuit and throw in the fact that the actual potential
of the earth at the grounding point is only arbitrarily zero, is
it any wonder when we find residual voltage on worksurfaces formerly
considered safe? High-resistance or intermittent grounds can sometimes
be as dangerous to sensitive devices as the total lack of ground
and can lead to a false sense of security.

Controlling high resistance to ground in a production situation
is simple compared to controlling it in a field-service environment.
At least you can run overall resistance tests on your building ground
circuit, and you can periodically recheck it. But a field-service
person can't exercise this control from the forty-fifth floor of
an office building.

Equipotential (Common) Grounding

To date, equipotential bonding has
been the answer. Equipotential bonding eliminates the risk of ESD
damage and may, in fact, bring an entire system to some common non-zero
potential. However, a good ground may still be necessary in some
situations to reduce residual voltage to an acceptable level in
an acceptable time.

The most accessible earth ground in an office is the equipment ground
of the electrical system in the building. But the first thing a
field-service person usually does when servicing a unit is to unplug
the unit from the building's electrical supply. This, in effect,
disconnects the potentially best available earth ground in the area.
The unit under service may now become a mass completely isolated
from ground by rubber insulating pads on leveling legs, on a massive
insulative plane of carpet.

The field-service person, under these conditions, may have established
a floating ground at a different potential form any of the AC-powered
equipment he may use while on site. This same condition has been
found in production plants where separate static -control grounds
and equipment power grounds are used.

Ten Rules
For Grounding Field-Service Personnel

The following
ten rules are for grounding a field-service person who wishes
to use a building's equipment ground in order to reduce residual
voltage to the lowest level.
1. Use only soft grounds, and check them regularly.
2. Watch out for parallel paths that may bypass the soft ground.
3. Avoid connecting wrist straps, ground cords and worksurfaces
in series.
4. Do not depend entirely on equipotential bonding for protection
when zero potential grounding is required. Use the system
described in this article.
5. Do not depend on a building's water pipes for electrical
earth grounding. Water supply lines may contain a length of
nonconductive pipe, or they may have insulative, sound-absorbing
couplings.
6. Remember that resistance to earth ground may increase as
you move to higher floors in an office building, or as you
move further away from the point where a building's equipment
ground is actually located.
7. Never assume that a wall plug has been properly wired.
Check with a circuit tester before connecting yourself to
a wall plug via wrist strap and ground cord.
8. Connect your entire ground system before touching any static-sensitive
device.
9. If you have a good field meter, check for the presence
of charged insulators, and move them at least 1 to 3 ft from
your work area. Remember that you should be grounded when
using the meter.
10. Use a properly grounded field service mat as your worksurface.

A Potential Solution

Before I adopted a modified equipment-ground
cord, residual voltage always disrupted the field-training demonstrations
I conducted in high-rise hotels or office buildings. Materials that
should have drained a charge to near zero in less than 1 sec might
show a residual voltage of more than 100 V after 30 sec or more.
Field meters that would normally return to zero would not do so
even when equip-potentially bonded to the operator and the conductive
worksurface being used in the demonstration area.

These problems were resolved by fabricating
a substitute equipment ground cord. It consists of a standard table
mat ground cord, with a 1- MOhm resistor, and a standard three-prong
electrical wall plug. The spade of the plug, which would normally
mate with the "hot" (phase) slot of a standard three-wire
wall receptacle, was removed. The ground cord with resistor was
connected to the equipment ground terminal on the plug. The normal
spade used for the neutral return remained unconnected and was left
in the plug to provide mechanical stability for connection to any
standard three-wire wall receptacle.

Since one can never assume that a given wall receptacle has been
properly wired, several commercial circuit testers such as an ECOS
Model-7106 ACCUTEST or a Pilgrim GAM-2 should be used. A unit of
this type will test for neutral-ground shorts (ground loops), neutral-ground
reversals, ground-path impedance and common wiring errors.

Alternatively, with a little extra effort , a simple VOM can be
used. However, circuit testers that simply check for proper wiring
at a wall plug without testing resistance to ground are inadequate
and should not be relied upon.

For added personal protection against electrical shock or possible
electrocution, when using a building ground as an ESD ground, a
short extension cord consisting of a heavy-duty plug, 12-AWG wire,
and an insulated box with cover containing a ground-fault circuit
interrupter can be fabricated from components available at any hardware
store. Should a dangerous overcurrent situation develop, the GFCI
will provide added measure of safety.

MIL-STD-883
Waveform Change Affects ESD Simulators

A subtle change was made to MIL-STD-883,
Method 3015, in Change Notice 7. And although the notice was
dated February 12, 1988, it didn't reach users until the middle
of the summer vacation season, so you may have missed it or
its significance. The change will require modification or replacement
of most of the human-body-model ESD simulators now in use.
Since the initial release of MIL-STD-883, the "ESD-Classification
Test-Circuit Waveform (human body model)" has been specified
in terms of voltage vs. time (see Fig 6). With Change Notice
7, the waveform is now specified in terms of current vs. time
and is also more complex.
Naturally, the concomitant waveform-verfication procedure changes
as well, as does the equipment needed to verify the waveform
and to make tests applying it. In particular, the change could
have a major impact on test equipment. It will require either
significant modifications to human-body-model testers or replacement
of units that can't be altered. Depending on the tester involved,
the cost to end users could be as little as a few hundred dollars,
or could go much higher.

Figure 6. Charge decay
is exponential (a), but if there is sufficient resistance between
the charged object and ground, a residual charge may remain
for a very long time (b). These are voltage waveforms; MIL-STD-883
now specified a current waveform (see sidebar).

Figure
7. All of the series resistances between worksurfaces (as well
as other objects) and ground must be known and considered.

Hooking up the Mobile Ground System

Beginning at a wall receptacle of a
building, here is the step-by-step procedure used to eliminate residual
voltage in a high rise building:

1. Test a convenient electrical receptacle
close to the worksite with one of the circuit testers mentioned
earlier to make sure the receptacle is properly wired and that overall
resistance to ground is appropriate.
2. Connect the short extension cord containing the GFCI to the outlet.
Test the GFCI to make sure it is working.
3. Lay out the field-service or floor mat.
4. Test the resistors in all of the ground cords using a good ohmmeter.
5. Using stackable snaps, connect one end of all of the ground cords
to one of the studs on the field-service mat.
6. Connect any AC-powered test equipment and/or the unit under repair
to one of the outlets of the GFCI in the short extension cord.
7. Plug the modified three-prong plug of the mat's ground cord into
the other receptacle of the GFCI.
8. If the unit under repair is unplugged from a power source, connect
the alligator clip of the mat-to-equipment ground to a good ground
on the frame of the unit.
9. Slip the wrist strap cuff onto your wrist, and tighten or adjust
it.
10. Finally, snap the resistor end of the pretested cuff-to-ground
cord onto the cuff.
Barring other unimpeded paths to ground, this system now provides
at least 2 MOhms of resistance to ground form both the operator
and the unit under repair, and at least 1 MOhm of resistance to
ground between the mat and ground. With common resistance to ground
from both the operator and the unit under repair, the operator and
the unit are equipotential.

Summary

While even the system described above
may not reduce residual voltage to zero in all cases, it is probably
the best alternative for the field-service person who must work
in various locations. As devices become more sensitive, this system
may become mandatory in order to reduce residual voltages as low
as possible.